1. Field of the Invention
The present invention relates to an ink, an ink jet recording method, an ink cartridge, a recording unit and an ink jet recording apparatus.
2. Description of the Related Art
An ink jet recording method is a recording method that minute droplets of an ink are applied to a recording medium such as plain paper or glossy medium, thereby forming an image, and is rapidly spread because of lowered price of an apparatus itself and improvement of recording speed. With the rapid spread of digital cameras in addition to the progress of the formation of high-quality images by the ink jet recording method, the ink jet recording method becomes general as an output method of images comparable with a silver halide photograph.
In recent years, there has been a demand for outputting a recorded article having image quality comparable with that of a silver halide photograph, in particular, a recorded article with reduced feeling of grain, excellent gradation property, which has image quality better than ever, with the spread of ink jet recording apparatus.
In order to solve such a problem, it is conducted to use a gray ink having a neutral color tone in addition to conventional respective inks of cyan, magenta and yellow. It is known that the use of the gray ink in addition to the conventional inks improves, for example, a feeling of grain in a shadow portion such as a flesh color or background portion, or gradation property upon gradation from a highlight to a shadow to obtain a recorded article having higher image quality. Incidentally, the meaning that an ink has a neutral color tone will be described subsequently.
A gray ink is generally prepared by conducting toning with coloring materials of cyan, magenta and yellow. It has been proposed to improve image quality by using the gray ink prepared by using such coloring material in combination. For example, it has been proposed to provide a light gray ink, a medium gray ink and a dark gray ink as toned gray inks to conduct gradation recording by combination of these inks (see Japanese Unexamined Patent Publication No. 2006-526062). This publication also discloses that a black coloring material is mixed with a proper coloring material to provide an gray ink.
On the other hand, the gray ink by the combination of cyan, magenta and yellow involves a problem that the image-storing ability of the resulting recorded article is low compared with the silver halide photograph. Specifically, when the recorded article formed with the gray ink is exposed to light, humidity or an environmental gas present in the air for a long period of time, a problem that a coloring material on the recorded article is deteriorated, and so change of color tone of the image or color fading is easy to occur arises. A large number of proposals has heretofore been made for improving the ozone fastness and light fastness of the image in particular in this image-storing ability.
The color fading of the image is mainly caused by the cyan ink particularly low in the ozone fastness among the respective inks of cyan, yellow and magenta. Therefore, a large number of proposals has been made for improving the ozone fastness of the cyan ink (see Japanese Patent Application Laid-Open Nos. 2002-249677, 2002-275386, 2002-294097, 2002-302623, 2002-327132, 2003-003099 and 2003-213168). In addition, a proposal has been made on the improvement in the ozone fastness of an image by introducing a nitrogen-containing aromatic heterocyclic ring into the skeleton of a phthalocyanine coloring material commonly used as a coloring material of the cyan ink (see Japanese Patent Application Laid-Open No. 2003-34758). It is described that this phthalocyanine coloring material may have (SO3D)m or (SO2NHR)n (m=1 to 4, n=0 to 3) as a substituent, and SO2NHR is SO2NH2 or a sulfonamide residue capable of forming a complex with a copper ion. Incidentally, D is a monovalent alkali metal, ammonium or organic ammonium.
Japanese Patent Application Laid-Open No. 2003-034758 describes that SO2NH2 is favorable as SO2NHR. However, three Examples among five Examples in Japanese Patent Application Laid-Open No. 2003-034758 are examples where the phthalocyanine coloring material has no SO2NHR, i.e., n is 0. The results of a color fading test with ozone gas when phthalocyanine coloring materials having SO2NH2 as SO2NHR and substituted by a sulfonamide residue were respectively used are described. However, it is disclosed that the results of the color fading test when these phthalocyanine coloring materials were used are poorer than the case of n=0 though Japanese Patent Application Laid-Open No. 2003-034758 describes that SO2NH2 is favorable as SO2NHR as described above.
The performance required of a recorded article obtained by the ink jet recording method for the ozone fastness is increased year by year, and so the coloring material heretofore used in the cyan ink does not come to provide an image having ozone resistance on the level satisfying the above requirement. For example, in the inventions described in Japanese Patent Application Laid-Open Nos. 2002-249677, 2002-275386, 2002-294097, 2002-302623, 2002-327132, 2003-003099, 2003-213168 and 2003-034758, it is attempted to improve the ozone fastness of the image by introducing various substituents into the coloring material. However, there is a limit to improve the ozone fastness of the image by only the properties of such a coloring material.